The present invention generally relates to a light-receiving element array, particularly to a light-receiving element array in which the crosstalk between light-receiving elements is decreased to prevent the degradation of a characteristic thereof.
Referring to FIG. 1, there is shown a commercially available light-receiving element array used in an optical demultiplexer for demultiplexing a wavelength multiplexed light and monitoring a spectrum of demultiplexed light. The light-receiving element array is structured by arranging light-receiving elements 10 in a straight line. Electrodes of respective light-receiving elements are connected alternately to bonding pads 12 positioned at both sides of the light-receiving element array.
A light-receiving element implementing a conventional light-receiving element array is a photodiode of pin-structure in which a pn-junction (this region constitutes a light-receiving area) is formed by diffusion. FIG. 2 shows an enlarged partially cross-sectional view of the array taken along the A-Axe2x80x2 line in FIG. 1. An n-InP layer (a buffer layer) 22, an undoped (i-) InGaAs layer (a light-absorbing layer) 24, and an n-InP layer (a window layer) 26 are stacked in this order on an n-InP substrate 20. Zn is diffused into the n-InP layer 26 (the diffusion is isotropic so that Zn is laterally diffused) to form a p-type diffused region 28, resulting in a pin-photodiode. In an optical demultiplexer using such a light-receiving element array, each of the demultiplexed lights is required to be entered to the light-receiving area of a corresponding light-receiving element.
In the conventional diffusion-type light-receiving element array, the elements are not isolated to each other, so that a part of the carriers generated in the light-absorbing layer by light absorption migrate to adjacent light-receiving elements by lateral diffusion. This migration of carriers causes the crosstalk toward adjacent light-receiving elements, resulting in the degradation of characteristic for the light-receiving element array.
For example, when an incident light impinges upon the peripheral portion of the diffused region 28 of a light-receiving element as shown in FIG. 3, carriers 27 are generated in a depletion layer 25 under the diffused region. The carriers diffuse outward from the depletion layer just under the diffused region as shown by an arrow 29. The carriers reached to the depletion layer of an adjacent light-receiving element causes the crosstalk.
As a relatively large electrical field is in the depletion region 25, the carriers generated in the depletion region, also, migrate downward along the electrical field. However, if the depletion region 25 is shallow and does not extend to the deep region of the light-absorbing layer 24 as shown in FIG. 3, the carriers are going to laterally diffuse because the electric field is small outside the depletion region, so that the crosstalk is also caused.
In order to cause the depletion region 25 to reach the buffer layer 22, it is preferable that a large reverse bias voltage is applied thereto. However, the depletion region is difficult to be extended, when the carrier concentration in the light-absorbing layer 24 is high.
If the incident light is more spread than the light-receiving area or the incident light partly enters outside the light-receiving area, the light impinges upon the area between light-receiving elements. The light impinged upon outside the light-receiving area (hereinafter referred to as a stray light) causes to generate carriers in the non-depleted light-absorbing layer between light-receiving elements. The carriers laterally diffuse and migrate to adjacent light-receiving elements, resulting in the crosstalk.
If the crosstalk described above is generated, the demultiplexed lights are difficult to be detected precisely, resulting in the degradation of characteristic of a light-receiving element array.
An object of the present invention is to provide a light-receiving element array in which the degradation of characteristic thereof due to the crosstalk may be prevented.
A first aspect of the present invention is a light-receiving element array comprising a plurality of light-receiving elements arrayed in a straight line, each light-receiving element being a pin-photodiode having a p-type or n-type layer formed by diffusion; and a light-shielding film provided on the top surface of the light-receiving element array except at least a part of light-receiving area of each light-receiving element.
A second aspect of the present invention is a light-receiving element array comprising a plurality of light-receiving elements arrayed in a straight line, each light-receiving elements being a pin-photodiode having a p-type or n-type layer formed by diffusion; each light-receiving element constitutes a mesa-structure with the light-receiving elements being isolated to each other by isolation trenches; and a light-shielding film provided on the top surface of the light-receiving element array except at least a part of light receiving area of each light-receiving element.
A third aspect of the present invention is a light-receiving element array comprising a plurality of light-receiving elements arrayed in a straight line, each light-receiving elements being a pin-photodiode formed by crystal growth; each light-receiving element constitutes a mesa-structure with the light-receiving elements being isolated to each other by isolation trenches; and a light-shielding film provided on the top surface of the light-receiving element array except at least a part of light receiving area of each light-receiving element.
A fourth aspect of the present invention is a light-receiving element array comprising a plurality of light-receiving elements arrayed in a straight line, each light-receiving element being a pin-photodiode formed by critical growth; wherein each light-receiving element constitutes a mesa and waveguide-structure with the light-receiving elements being isolated to each other by isolation trenches.
A fifth aspect of the present invention is a light-receiving device, comprising:
a light-receiving element array including a plurality of light-receiving elements arrayed in a straight line, each light-receiving element being a pin-photodiode formed by critical growth, each light-receiving element constituting a mesa and waveguide-structure with the light-receiving elements being isolated to each other by isolation trenches; and
a circuit board on which the light-receiving element array is mounted, the circuit board including,
a pattern of electrode wirings which are formed in the same pitch as that of the second conductivity-type of electrodes,
a plurality of first leads for the pattern of electrode wirings,
a plurality of first bonding pads connected to the first leads, respectively,
one second bonding pad provided near the light-receiving element array on the circuit board,
a second lead for the second bonding pad, and
a third bonding pad connected to the second lead,
wherein the second conductivity-type of electrodes are connected to the pattern of electrode wirings, and the first conductivity-type of electrode is connected to the second bonding pad.